The correct answer is C. Add the standard Chatter feed component.
To add Chatter feed functionality to a custom object Lightning record page, the app builder should use the standard Chatter component in Lightning App Builder. Salesforce documentation states that the Feed component was renamed Chatter in Lightning App Builder and is used to place the publisher and feed combination on a record page.
A is incorrect because no custom Chatter feed component is required. B is incorrect because related lists do not add the Chatter feed. D is incorrect because the standard Chatter component is already available; an AppExchange component is unnecessary.

A Screen Flow is the best declarative tool when the user must answer questions through a guided UI and use those answers to update record fields. Salesforce documents that screen flows provide a UI that guides users through a business process, and they can be launched from quick actions or Lightning pages.
A custom button or action can launch the flow, collect the extra approval-related information, update the Account fields, and then continue the approval submission logic.
Why not others:
A). Custom object only stores data; it does not provide the popup/question flow.
B). Visualforce could be custom-coded, but Flow is the declarative App Builder solution.
D). Custom Lightning component requires development and is not the best declarative answer.

The correct answer is C. Apex unit test failures.
Because the unmanaged package was successfully installed in a Full Copy sandbox that matches production, issues like feature incompatibility, license mismatch, or object limits would normally have appeared during sandbox testing. The remaining Salesforce-specific production blocker is Apex test execution.
Salesforce deployment documentation states that if any Apex test fails, the deployment is rolled back.
Salesforce Apex testing guidance also states that Apex unit tests must meet coverage requirements and complete successfully for deployment or packaging scenarios.
So the package failed in production because the required Apex tests did not complete successfully.

The correct answer is D. Approval process.
The requirement is to automate travel request form submission and enforce an internal approval policy.
Salesforce approval processes are specifically designed to automate how records are submitted, reviewed, approved, rejected, recalled, and acted upon. Salesforce Trailhead explains that an approval process defines the criteria a record must meet before submission, the approval steps, approvers, and the actions taken when the record is approved or rejected.
This exact scenario also aligns with Salesforce's travel approval training example, where a Travel Approval object uses an approval process to route travel requests to the employee's manager and, when needed, to another approver.
A is incorrect because assignment rules route leads or cases, not approval requests.
B is incorrect because validation rules prevent invalid saves but do not manage approval routing.
C is incorrect because Apex is unnecessary when a standard declarative approval process fits the requirement.

The correct component is the Highlights Panel. More precisely, Salesforce refers to the newer version as the Dynamic Highlights Panel, which displays up to 12 key record fields configured in Lightning App Builder.
Since the option list only says Highlights Panel, option B is the exam match.
Path is for guiding users through stages. Accordion is for collapsible sections. A custom Lightning Web Component is not needed because Salesforce provides this functionality declaratively.